The long-term objective is to delineate the basic principles governing synaptic connectivity in central neurons and the significance of these mechanisms to information storage as well as brain dysfunction underlying mental illnesses. Dissociated hippocampal neurons form functional synapses when grown in culture. Electrophysiological and imaging methods will be applied to investigate the cellular and molecular mechanisms of long-term depression (LTD) of synaptic strengths and the maintenance of the relative balance of synaptic inputs in simple circuits formed by cultured hippocampal neurons. I. Mechanisms of LTD in simple circuits: Culture preparation can provide answers to mechanisms of synaptic connectivity that cannot be addressed in brain slices. The first aim is to demonstrate the significance of LTD in culture by establishing its stability, pharmacological requirements, and the presynaptic mechanisms of LTD. II. Properties of LTD expression at individual synapses in simple circuits: The neuronal culture preparation will be fully exploited by using the activity-dependent fluorescent synapse marker FM1-43 to study LTD at the level of individual synapses. In particular, the spatial pattern of LTD expression and structural correlates of LTD at individual synapses will be characterized. III. Mechanisms regulating synaptic strengths in two cell circuit: Again, making use of the culture preparation, the regulation of relative strengths of synaptic inputs will be addressed. Specifically, the origin and the underlying mechanism leading to asymmetric synaptic inputs in reciprocally interconnected pairs of cultured hippocamal neurons will be studied.